Kinetics of the reactions of benzylidene Meldrum's acid with thiolate and alkoxide ions in aqueous dimethyl sulfoxide

A kinetic study of the reaction of benzylidene Meldrum's acid, PhCH=C(COO)(2)C(CH3)(2) (5 . H), with a series of thiolate and alkoxide ions in 50% DMSO-50% water (v/v) at 20 degrees C is reported. The reactions with RX- (X = S or O) lead to adducts of the type PhCH(XR)C(COO)(2)C(CH3)(2)(-) ((5-H,XR)(-)), which can be viewed as a model for the intermediate of a nucleophilic vinylic substitution on substrates such as PhC(LG)=C(COO)(2)C(CH3)(2) (LG = leaving group). Our measurements allowed a determination of rate and equilibrium constants for these processes with RS- = n-BuS-, HOCH2CH2S-, MeO2CCH2CH2S-, and MeO2CCH2S- and RO- = OH-, MeO- (only rate constant of breakdown of adduct), HC=CCH2O-, and CF3CH2O-. Our results show that there are major differences between the alkoxide and thiolate ions with respect to their thermodynamic and kinetic affinities to 5-H. They arise mainly from differences in the polarizability and solvation between the sulfur and the oxygen bases. Similarities and differences between the reactions of thiolate ions with 5-H and alpha-nitrostilbenes (4-H) are also discussed.